Spinal Instability and Pain: Is There a Connection?

By Craig Liebenson, DC

A new study examined the theory that antagonistic trunk muscle coactivation is necessary to provide mechanical stability to the lumbar spine around a neutral posture. The authors found that antagonistic muscle coactivation increased in response to increased axial load on the spine.

The subjects were asked to perform slow trunk flexion and extension movements in a semi-seated position with hip motion restricted, but trunk motion free. Weights were then added to the torso. One conclusion was that "increased levels of muscle coactivation may constitute an objective indicator of the dysfunction in the passive stabilizing system of the lumbar spine."

Other recent studies have looked at various aspects of motor control and evaluated their correlation with pain and injury.

Reaction Time

When comparing back pain patients to asymptomatic subjects, the back pain patients had a slower reaction time, decreased peak output, increased after discharges when irregular load is handled. This study had particularly strong methodology because treatment was given and the reactions improved. Then sitting was shown to disturb these variables and a brief walking break to improve them again.

Coordination

Loss of control of center of rotation during isoinertial resisted trunk movements in the sagittal plane occurred in low back pain patients, but not in normals. Increase in rotation and side bending and decrease in sagittal motion occurred.

Spinal loading forces were increased during a fatiguing isometric trunk extension effort without a loss of torque output. Torque output remained constant because as the erector spinae fatigued, substitution by secondary extensors such as the internal oblique and latissimus dorsi muscles occurred.

Overactivity of antagonist back muscles during the ipsilateral swing phase of gait and decreased agonist peak muscle activity during double stance phase differentiated back pain patients from asymptomatics.

Altered muscle activation ratios of synergist spinal muscles during a variety of motor tasks differentiated from injured and uninjured individuals. Underactivity of agonists and overactivity of synergists was able to discriminate pain patients with 88% accuracy.

Endurance

Decreased endurance of the trunk extensors has not only been shown to correlate with pain, but to predict recurrences and first time onset in healthy individuals. This evidence is extremely strong because it is prospective and thus the findings are not merely correlated by association, but by etiology.

Atrophy

The multifidus in the low back has been shown to be atrophied in patients with acute low back pain, those recovered from acute low back pain, and those having surgery for nerve root compression. The acute patients' atrophy was unilateral to the pain and at the same segmental level as palpable joint dysfunction. Recovery from acute pain did not automatically result in restoration of the normal girth of the muscle. However, spinal stabilization exercises successfully did rebuild the muscle's size.

According to Edgerton et al., "The nervous system apparently can detect a reduced capacity to generate force from a specific muscle or group of muscles and compensate by recruiting more motoneurons. This compensation can be made by recruiting motor units from an uninjured area of the muscle or from other muscles capable of performing the same tasks ..."

According to Korr, "The brain thinks in terms of whole motions, not individual muscles."